Ultrasonically Welded Fuel Cell Unitized Electrode Assembly

ABSTRACT

A unitized electrode assembly ( 9 ) for use in the fuel cell comprises a first GDL ( 23 ), a PEM ( 28 ), and a second GDL ( 12 ), with electrode catalyst ( 27, 30 ) disposed between said PEM and each of said GDLs, said layers ( 23, 27, 30, 12 ) being impregnated with a thermoplastic polymer a sufficient distance from each edge of the UEA so as to form a fluid seal ( 13 ). The UEA is formed by a process which comprises making a sandwich of some or all of said layers ( 23, 27, 28, 30  and  33 ), with thermoplastic polymer film ( 22, 25, 32, 35 ) extending inwardly from the edges of said sandwich a sufficient distance to form the seal, said thermoplastic polymer film being disposed between each electrode and the adjacent GDL and/or between each GDL and release film ( 21, 36 ) on the top and bottom of the sandwich. The sandwich, compressed by force between a holding fixture ( 19 ) and an anvil ( 40 ), is heated by ultrasonic vibration so as to cause said thermoplastic polymer to melt and impregnate the layers ( 23, 27, 30, 12 ) of the UEA with the film ( 22  and/or  25; 32  and/or  35 ).

TECHNICAL FIELD

This invention relates to a unitized electrode assembly (UEA) for a fuelcell which is produced by utilizing a film of thermoplastic polymer,such as polyethylene, polypropylene, or other easily melted high flowingplastic, layered around the cathode and anode gas diffusion layers(GDLs). Ultrasonic welding, such as vertical vibrational pressure, isutilized to heat the edge areas of the UEA planform, causing the plasticto diffuse through the GDLs and the electrode catalysts, thereby forminga solid plastic edge serving as a fluid seal of the UEA.

BACKGROUND ART

Fuel cell power plants that employ a polymer, proton exchange membraneelectrolyte (PEM) include a cathode electrode on one side of the PEM andan anode electrode on the other side of the PEM, the electrodescomprising suitable catalysts so as to convert hydrogen and oxygenreactant gases into electricity and water, all as is known. Thereactants reach the membrane by means of reactant gas flow field plates,sometimes referred to as water transport plates, and thence gasdiffusion layers (GDLs), occasionally referred to as substrates. TheGDLs are adjacent to respective sides of the electrodes. The membranemay typically be a fluorinated polymer, such as that sold under the nameNAFION®. The electrodes are typically a mixture of polymer and noblemetal, as is known.

A recent innovation is to manufacture a unitized electrode assembly(UEA), including the anode and cathode GDLs and electrodes, onrespective sides of the membrane, unitized and sealed into a singlestructure, using thermoplastics. Thermoplastics only undergo a change ofstate (liquefy) when at a high temperature and return to a solid statewhen cooled, and can be re-melted and reformed. This is in contrast withthermoset plastics, which, when formed, undergo an irreversible chemicalchange, and cannot be reformed with heat.

Techniques for joining thermoplastics use localized heating of thethermoplastics to be joined causing melting, followed byresolidification at the interface.

In “frictional welding”, moving one part against the other generatesheat at the interface causing one or both parts to melt. Once meltingbegins, the parts are held together until the thermoplastics solidify toeach other. This method may also be known as “linear vibration welding”,“orbital vibration welding”, or “spin welding”.

“Laser or IR” welding directs a beam of laser or IR through atransparent thermoplastic causing surface heating of an opaquethermoplastic at the interface of the two thermoplastics. When theinterface reaches a sufficient temperature, the plastics begin to meltand bond together by interflow.

“Radio Frequency” welding, also called “High Frequency” welding relieson the dissipation of some of the energy of a changing electromagneticfield in an imperfect dielectric to heat the plastic; subsequent coolingcauses two plastics to be joined together.

In “Hot Plate” welding methods, one or both of the plastic pieces to bejoined is/are held against a hot plate until softening begins. Theplastic is removed from the hot plate and placed against the matingsurface and held until cooled.

The welding methods described are not useful in the manufacture of MEAsbecause they only effect the joining of two thermoplastic surfacestogether at their interface.

“Ultrasonic welding” is defined as employing mechanical oscillationsbetween 16 kHz and 1 GHz. Typical ultrasonic welding machines operate inthe 15 kHz to 70 kHz range and most commonly around 20 kHz. A generatorproduces electrical oscillations at the desired frequency, which arethen transferred to a converter in which crystals expand and contractcreating mechanical vibrations at the same frequency. These vibrationsare transferred to a horn that contacts the stack of plastic parts to bewelded. As the horn moves vertically up and down, perpendicular to theplane of the parts, heat friction develops along the joining areabetween the two plastic parts that melts the plastic and joins theparts.

Not all thermoplastics respond the same to ultrasonic welding. Thosethat have an amorphous polymer structure, characterized by randomarrangement of molecules, will have a broader softening and meltingpoint and transfer ultrasonic vibrations well. Examples of suchthermoplastics are polystyrene, polyetherimide and low densitypolyethylene. Thermoplastic polymers of a semi-crystalline nature havemore ordered structure and well-defined melting points and do nottransfer ultrasonic vibrations as well and are therefore harder to weld.Examples of such thermoplastics are polyester, polyethylene, and linearlow density polyethylene (LLDPE). In general, high melting point and lowmelt index polymers are more difficult to weld.

A “press plate” method for manufacturing a UEA involves laying out acomplete UEA with polyethylene films between the various layers and onthe exterior of the GDLs. Then, press plates apply pressure to theassembly as the press plates are heated to on the order of 150° C. (320°F.). Thereafter, the press plates must be cooled before pressure isreleased and the sealed UEA removed from the press plates. This processtypically takes at least ten and as much as sixty minutes per UEAmanufactured. The process is costly and consumes manufacturing floorspace. In addition, the process is inefficient in that it requiresheating of the entire planform in addition to the press plates. It isknown that elevated temperature causes degradation of the PEM, and thusthe durability of the UEA is reduced as a consequence of heating of theentire planform of the UEA during manufacture.

Another method utilizing injection molding or compression molding of athermoplastic polymer is disclosed in patent application PCT/US03/01796,International Publication No. WO03/063280 A2. This process may requirepre-treating such as corona treatment, oxygen plasma treatment orfluoropolymer dispersions. There are additional problems of thefountain-flow thermoplastics readjusting the positioning of components,and the like. These and other problems require additional processingtechniques in order to cause successful manufacture of UEAs.

DISCLOSURE OF INVENTION

Objects of the invention include: a method of manufacturing a fuel cellUEA which heats only the outside edge of the UEA which is to be sealed;providing a UEA without subjecting the PEM to elevated temperatures;providing manufacture of a UEA in short cycle times; avoiding thenecessity of heating and cooling press plates, as well as the entireUEA, in the manufacture of a UEA; manufacturing UEAs for fuel cells witha single, exclusive method; and improved fuel cell UEAs.

This invention is predicated on the discovery that thermoplasticpolymers, such as polyethylene, when heated by vibrational energy, willimpregnate carbon fiber of gas diffusion layers and diffuse into theporous catalyst/polymer electrode layers adjacent to the PEMs, therebyforming a solid plastic seal of the entire permeable edge volume of aUEA.

According to a first form of the present invention, an easily melted,free-flowing thermoplastic polymer film, such as polyethylene film, isplaced outside of the GDLs and/or between each GDL and the relatedelectrode along the edges of the periphery of a laid-up UEA sandwich, asuitable non-melting release film being placed on the upper and lowersurfaces of the UEA sandwich; the thermoplastic polymer film is meltedby vibrational energy applied only to the edges of the UEA sandwich,that is, adjacent to the plastic film; the vibrating energy melts thefilm and pressure applied at the same time causes the thermoplasticpolymer to impregnate the GDLs, and diffuse into the adjacent porouscatalyst layers, forming a mechanical bond therewith.

In accordance with the invention in another form, the edges of the GDLsare impregnated with a thermoplastic and then laid up in a sandwichadjacent to the electrodes, with the PEM between the electrodes andrelease film on the upper and lower surfaces. The vibrational energy isapplied with compression to form the UEA, as in the first form of theinvention described hereinbefore, using the thermoplastic preformed intothe GDLs.

In one form of the invention, the UEA is formed one edge at a time by anultrasonic welder having a straight vibrating anvil; in another form,the invention may be practiced utilizing an ultrasonic welder having aframe-shaped vibrating anvil. Other combinations of edges may also bewelded using this invention.

The invention avoids subjecting the active area of the UEA to elevatedtemperature which might reduce the durability of the polymer exchangemembrane.

The melting, impregnating, diffusion and cooling to make a UEA accordingto the invention requires only on the order of five seconds. Theinvention permits cycle times on the order of one-half minute to severalminutes, and uses less energy than prior art assembly methods.

The invention may be used to make unitized assemblies forelectrochemical cells other than fuel cells, such as electrolyzers.

Other objects, features and advantages of the present invention willbecome more apparent in the light of the following detailed descriptionof exemplary embodiments thereof, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a unitized electrode assembly made inaccordance with the process of the present invention.

FIG. 2 is a partial, sectioned side elevation view, with sectioninglines omitted for clarity, taken on the line A-A in FIG. 1, beforeunitization.

FIG. 3 is a partial, sectioned side elevation view, with sectioninglines omitted for clarity, of the section of FIG. 2 when the UEA iscompleted.

FIG. 4 is a partial, sectioned side elevation view, with sectioninglines omitted for clarity, of a first alternative to the method of FIGS.2 and 3.

FIG. 5 is a partial, sectioned side elevation view, with sectioninglines omitted for clarity, of a second alternative to the method ofFIGS. 2 and 3.

FIG. 6 is a partial, sectioned side elevation view, with sectioninglines omitted for clarity, of a second form of the invention.

MODE(S) FOR CARRYING OUT THE INVENTION

Referring to FIGS. 1 and 2, a unitized electrode assembly 9 made inaccordance with the process of the present invention includes a reactantgas flow distribution layer herein referred to as a gas diffusion layer(GDL) 12, and an integrated seal 13; holes 15, 16 register the variouslayers during the process of forming the seal 13. The seal 13 extendsaround the edge of the entire periphery of the unitized electrodeassembly 9. In a typical case, the unitized electrode assembly might beon the order of 8 cm by 14 cm, and the seal may be formed to have awidth of 2 cm-3 cm when formed, and then the UEA may be trimmed so thatthe seal width is on the order of 8 mm-10 mm.

The process comprises laying up the various components as illustrated inFIG. 2, above a holding fixture, which has pins to receive theregistering holes 15, 16. There is first laid down a release film 21which may be TEFLON®, KAPTON®, or any other high temperature plasticfilm which will not melt or stick to the polyethylene, including a filmof high temperature polyethylene. Then there is a first layer 22 ofthermoplastic polymer film, which may extend a variety of distances fromthe edge sufficient to form a fluid edge seal; in this embodiment thefilm 22 may extend on the order of two or three centimeters in from theedge of the UEA assembly. A GDL 23, such as an anode GDL, is positionedabove the film 22. A second layer 25 of thermoplastic polymer film ispositioned above the GDL 23. An electrode, such as an anode electrode27, which comprises a conventional fuel cell catalyst, is disposed abovethe second layer 25 of film. The PEM 28 is disposed above the electrode27. Another electrode, such as a cathode electrode 30, is positionedabove the PEM 28. A third layer 32 of thermoplastic polymer film ispositioned above the electrode 30. A second GDL 12, such as a cathodeGDL, is disposed above the third layer of film 32. A fourth layer 35 ofthermoplastic polymer film is positioned above the second GDL 12. Arelease film 36 is positioned at the top of the stack of components.

The holding fixture 19 is part of an ultrasonic welding machine whichhas an anvil 40 that provides a vertical force as the distance betweenthe fixture 19 and the anvil 40 is varied ultrasonically, by on theorder of 2 micrometers to 10 micrometers. An available system forserving this purpose is the Branson 2000 IW Ultrasonic Welding System.The clamping force may be on the order of 500 kPa (60 psig). Thevibration may, for instance, be on the order of 20 kHz. The anvil may bestraight, so that each edge of the UEA is sealed separately, or have apicture frame shape so that four edges of a UEA may be sealed in asingle plunge. Other combinations of edge sealing may also be used.

After the force has been applied, with vibration, for about one-halfsecond, the polyethylene film becomes completely diffused through thevarious layers to form an integrated seal 13, completely impregnatedthroughout, as is illustrated by stippling in FIG. 3.

Instead of thermoplastic sheets (22, 25; 32, 35) placed on both sides ofeach GDL, thermoplastic sheets 25, 32 may be placed only between eachGDL 23, 12 and the adjacent electrode 27, 30, as shown in FIG. 4. Underpressure and vibration for a suitable time, the melted plastic willimpregnate the edge of each GDL from the inside out and diffuse into theelectrodes to provide the seal described with respect to FIG. 3hereinbefore.

Alternatively, the thermoplastic sheets 22, 35 may be placed outside ofthe GDLs 23, 12, as shown in FIG. 5. The melted plastic will impregnateeach GDL from the outside in.

In another form of the invention, shown in FIG. 6, GDL's 12 a, 23 a areimpregnated with thermoplastic polymer, as shown by the stippling, andthereafter laid up in a sandwich with the release film 21, the anodeelectrode 27, the PEM 28, the cathode electrode 30, and the release film36. Then the vibrating force is applied by the ultrasonic weldingmachine 19, 40.

The thermoplastic polymer may comprise polyethylene, polypropylene orother suitable polymers.

1. A method of preparing a fuel cell unitized electrode assembly (9)having a plurality of edges, which method is characterized by: forming amulti-layer sandwich comprising a proton exchange membrane electrolyte(28) having a pair of opposed surfaces, a pair of electrode catalystlayers (27, 30), each disposed adjacent a corresponding one of saidsurfaces, and a pair of gas diffusion layers (23, 12; 23 a, 12 a) eachhaving an upper surface and a lower surface, and each having athermoplastic polymer either (a) dispersed therein or (b) in a layer ofthermoplastic polymer film adjacent to (i) either said upper surface, or(ii) said lower surface, or (iii) both said upper and lower surfaces ofeach of said gas diffusion layers, said thermoplastic polymer extendinginwardly a distance from said edges to provide a fluid edge-seal aboutthe periphery of said unitized electrode assembly; and applying (19, 40)a clamping force and ultrasonic vibrational energy to said sandwich, toimpregnate said first and second electrode catalyst layers and be bondedwith thermoplastic polymer in said gas diffusion layers.
 2. A methodaccording to claim 1 wherein: said applying step comprises heating saidthermoplastic polymer with vibrational energy sufficient to cause saidthermoplastic polymer to bond said electrode catalyst layers with saidgas diffusion layers.
 3. A method according to claim 1 wherein: saidforming step is performed (a) on one edge at a time or (b) on more thanone edge at a time.
 4. A method of preparing a fuel cell unitizedelectrode assembly (9) having a plurality of edges, which method ischaracterized by: forming a multi-layer sandwich comprising a first gasdiffusion layer (23) having upper and lower surfaces, a first electrodecatalyst (27), either (a) a first thermoplastic polymer film (22)disposed on said upper surface of said first gas diffusion layer, or (b)a second thermoplastic polymer film (25) disposed on said lower surfaceof said first gas diffusion layer, or (c) both said first and secondthermoplastic polymer films, a proton exchange membrane electrolyte (28)having a first surface adjacent said first electrode catalyst (27), asecond electrode catalyst (30) adjacent a second surface of said protonexchange membrane electrolyte, a second gas diffusion layer (12) havingupper and lower surfaces, and either (d) a third thermoplastic polymerfilm disposed on said upper surface of said second gas diffusion layer,or (e) a fourth thermoplastic polymer film (35) disposed on said lowersurface of said second gas diffusion layer, or (f) both said third andfourth thermoplastic polymer films, said thermoplastic polymer filmsextending inwardly from said edges a distance to provide a fluidedge-seal about the periphery of said unitized electrode assembly; andapplying (19, 40) a clamping force and ultrasonic vibrational energy tosaid sandwich to cause said thermoplastic polymer to impregnate and bondsaid gas diffusion layers and said electrode catalysts.
 5. A methodaccording to claim 4 wherein: said forming step is performed (a) on oneedge at a time or (b) on more than one edge at a time.
 6. A methodaccording to claim 4 further characterized by: said sandwich beingdisposed between release films (21, 36) prior to applying said clampingforce and ultrasonic vibrational energy.
 7. A method of preparing a fuelcell unitized electrode assembly (9) having a plurality of edges, whichmethod is characterized by: forming a multi-layer sandwich comprising afirst release film (21), a first gas diffusion layer (23), a firstelectrode catalyst (27), either (a) a first thermoplastic polymer film(22) disposed between said first release film and said first gasdiffusion layer, or (b) a second thermoplastic polymer film (25)disposed between said first gas diffusion layer and said first electrodecatalyst, or (c) both said first and second thermoplastic polymer films,a proton exchange membrane electrolyte (28) having a first surfaceadjacent said first electrode catalyst (27), a second electrode catalyst(30) adjacent a second surface of said proton exchange membraneelectrolyte, a second gas diffusion layer (12), a second release film(36), and either (d) a third thermoplastic polymer film disposed betweensaid second electrode catalyst and said second gas diffusion layer, or(e) a fourth thermoplastic polymer film (35) disposed between saidsecond gas diffusion layer and said second release film, or (f) bothsaid third and fourth thermoplastic polymer films, said thermoplasticpolymer films extending inwardly from said edges to provide a fluidedge-seal about the periphery of said unitized electrode assembly; andapplying (19, 40) a clamping force and ultrasonic vibrational energy tosaid sandwich to cause said thermoplastic polymer to impregnate and bondsaid gas diffusion layers and said electrode catalysts.
 8. A methodaccording to claim 7 wherein: said applying step comprises heating saidthermoplastic polymer with vibrational energy sufficient to cause saidthermoplastic polymer to bond said electrode catalyst layers with saidgas diffusion layers.
 9. A method according to claim 7 wherein: saidforming step is performed (a) on one edge at a time or (b) on more thanone edge at a time.
 10. A fuel cell unitized electrode assembly (9) madeaccording to the process of claim
 1. 11. A fuel cell unitized electrodeassembly (9) made according to the process of claim
 4. 12. A fuel cellunitized electrode assembly (9) made according to the process of claim7.
 13. A fuel cell stack comprising a plurality of fuel cells accordingto claim
 10. 14. A fuel cell unitized electrode assembly unitized byultrasonic welding.